In a hydraulically controlled device such as a hydraulically driven vehicle, it is well known that a four-way valve is used to select forward and reverse directions and control speed. Using a meter-in version of such four-way valve, it is no problem to maintain the speed of such device on the level or uphill, but when going downhill, inertia and gravity will often cause such vehicle to go faster than desired, so that excessive braking is needed under such circumstances. Using a two port meter-out four-way valve, instead of a two port meter-in valve will, however, permit the user to control downhill speed, as well as level and uphill speed, by use of a four-way, two port meter-out valve.
There are of course many other uses for four-way valves, and, depending upon the desired use, such valves may need to be structurally different. For example, in U.S. Pat. No. 3,985,153 to Thomas a four-way two port meter-in valve is shown having an inlet port and a pair of active control ports for use with a fixed volume pump, whereby the flow from the pump goes to one action port and exhausts out the other active port, while at the same time having internal flow control spools for maintaining a constant pressure differential between the pressure in the fluid inlet passage and the pressure supplied to the active port being supplied. In the Thomas patent, the importance of having a steady controlled fluid flow and the desirability of incorporating the pressure compensating valve within the four-way valve is explained in detail.
Because of the need for various types of pressure compensated four-way valves, suppliers of such valves are required to stock the various valves needed from time to time by their customers. Consequently, there is a need for four-way valves which are convertible from one version to another, whereby a supplier can have one valve structure in stock, which can be supplied as is to purchaser if one type four-way valve is desired, or quickly converted to another type of four-way valve if such other type of valve is desired. Such valve structures allow the supplier to have less inventory on hand, thereby lowering his inventory costs, while at the same time having in stock the valves that such supplier's customers are likely to want.